1. Field of the Invention
The present invention relates to an optical film and to a method for producing it. Precisely, the invention relates to an optical film produced according to a specific production method and having a particular internal structure, and to a polarizer and a liquid crystal display device each having the optical film.
2. Description of the Related Art
With the recent prosperity of the liquid crystal display device market, various films have been developed. For example, JP-A 6-222213, 2003-25414 and 2007-38646 disclose tilted retardation films having a tilt structure.
For example, JP-A 6-222213 describes a method for producing a film of which the optical axis in the thickness direction is tilted, and the method comprises introducing a film between two rolls each running at a different peripheral speed to thereby impart a shearing force to the film, and stretching the film in the lateral direction (in the direction perpendicular to the film traveling direction) in a high temperature atmosphere; and the reference also describes application of the film to a TN-mode liquid crystal display device. However, the reference does not suggest impartation of a shearing force to a melt, but describes only impartation of a shearing force to a solid film. The method of imparting a shearing force to a solid film is problematic in point of the uniformity of the optical properties of the obtained optical film.
JP-A 2003-25414 and 2007-38646 describe a method comprising sandwiching a molten material between two rolls of a rubber roll and a metal roll of which the peripheral speed may differ from that of the rubber roll, thereby imparting a shearing force thereto to produce a film of which the optical axis in the thickness direction is tilted. JP-A 2003-25414 and 2007-38646 describe or suggest the applicability of the optical film described therein to a TN-mode liquid crystal display device. On the other hand, though these references say that it is preferable to stretch an optical film having a tilt structure, they disclose only examples of an embodiment of stretching a film in the machine direction (film traveling direction), or that is, no detailed investigation is made therein relating to the internal structure of an optical film obtained by stretching a film having a tilt structure in the machine direction.
On the other hand, regarding the alignment of the liquid crystal molecules in the liquid crystal cell in a TN-mode liquid crystal display device, it is known that the liquid crystal molecules are tilted in the thickness direction between electrodes and the alignment of the liquid crystal molecules changes continuously in some degree toward the thickness direction from one end to the other end of the liquid crystal cell. Assiduous studies have been made for the purpose of obtaining an optical film capable of optically compensating only the liquid crystal molecules thus aligned in that manner; however, the details of the internal structure of the obtained optical film are not disclosed in any references including the above-mentioned JP-A 6-222213, 2003-25414 and 2007-38646, and in fact, any detailed investigations relating to this have not been made as yet.